There will come a time during the life of most production wells in which the bottomhole pressure and gas to liquid ratio will not support natural flow. At this stage some form of artificial lift must be selected to remove liquids from the well so that production can continue.
One form of artificial lift entails repetitively closing in the well to allow pressure to buildup as liquid flows into the well. When a compatible combination of pressure and liquid accumulation is reached the well is opened and the liquid can be produced to a sales line and/or tank. Intermitting techniques include time based cycling and event based cycling. Time based cycling is based solely on a clock. The well is set to be flowing for a certain period of time and then shut-in for a certain number of hours. Event based cycling, optimization, is a method where certain inputs, such as pressures, flow rate, or differential pressure are set at a threshold value. When the threshold value is crossed an event occurs such as opening the well or shutting the well in. “Open triggers” are thresholds that trigger opening the well and “close triggers” are thresholds that trigger closing the well.
Plunger lift systems use intermitting techniques in combination with a free traveling plunger in the tubing string that provides an interface between the liquid phase and the gas phase. Use of a plunger facilitates operating the well at a lower flowing bottomhole pressure than non-plunger intermitting techniques, thereby enhancing well production.
In a typical plunger lift system, the well is completed with casing, tubing and a control valve. A sales line connects the control valve to the remainder of the gas distribution system and a sales meter is connected to the sales line for measuring the amount of gas that has passed through the sales line. Gas enters the well from the surrounding formation through perforations in the casing. Closing the control valve has the effect of allowing pressure inside the casing to increase. The tubing extends from the valve to near the bottom of the casing. A plunger is positioned on a bumper spring at or near the bottom of the tubing. After a fixed amount of time has past, or after the casing pressure or another trigger has crossed a particular threshold value, the valve is automatically opened and the plunger is forced upward due to the built up pressure inside the casing. Ideally, opening of the valve in this manner allows the gas, as well as any oil and water, to be forced up the tubing by the plunger. As long as the valve is open, more gas, and in many instances oil and water, flow into the tubing below the plunger. Once the plunger reaches the top of the tubing, gas flows through or past the plunger into a line. After a fixed amount of time has past, or after the casing pressure or another trigger crosses a particular threshold value, the valve is closed and the plunger returns back down the tubing, through the liquid, to stop on the bumper spring at or near the bottom of the tubing.
In many plunger systems a controller senses the gas pressure in the casing and opens the valve when the pressure exceeds a fixed value or a fixed amount of time has past. These plunger systems have a number of problems in the production of natural gas. If a controller blindly opens the valve when the casing pressure is deemed sufficient, or after a fixed amount of time has past since the last cycle, the valve may either be opened too early or too late in the cycle to optimize gas production. If the valve is opened too early, the pressure in the casing is insufficient to force the plunger to completely lift the water and oil out of the well. If this continues it can result in the well loading up with oil and water and shutting-in. In this case, gas production continues to decrease until it ceases, causing an interruption in gas production and a corresponding loss of revenues derived from that well.
In the situation where the valve is opened too late, excessive pressures can build up behind the plunger, forcefully impacting the plunger against the top of the casing and potentially causing damage. Even if no damage is done, waiting too long between opening the valve after each cycle means less gas is produced from the well, resulting in a corresponding loss of revenues derived from that well. In addition, when excessive pressure builds in the casing, the corresponding pressure differential between the tubing line pressure and the sales line pressure becomes great. In this situation when the valve is opened and the plunger rises at high speeds, the gas flow in the sales line exceeds the maximum measurable by the sales meter. Of course, there is a corresponding loss of revenues derived from the well when quantities of gas flow from the well into the sales line without being registered on the sales meter.
The characteristics of even a once perfectly tuned system change over time, causing the interval between opening the valve to be less than optimal. Accordingly, it is desirable to control a well plunger system in a manner that compensates for changing characteristics of the well.